There has been known a camera having an automatic focus control apparatus of the type in which a scanning mechanism of a photoelectric distance measuring instrument is made to make one round trip in two directions, and after a circuit is caused to memorize the maximum or minimum value (This is a value selected by the design of the circuit.) in the photoelectric signal detected in a scanning stage in one direction, and when in the scanning stage in the inverse direction a photoelectric signal almost equal to the maximum or minimum value memorized in the former direction is obtained, a camera lens in the moving state for focus regulation is stopped by a detection signal.
In the automatic focus control apparatus of such a camera, the lower the scanning speed of scanning mechanism of a photoelectric distance measuring instrument, the higher the accuracy of focus regulation becomes. Because usually a camera lens is coupled with scanning mechanism, etc. and caused to stop moving of the basis on a detection signal of the maximum value (or minimum value) from the distance measuring instrument, but since there is naturally some time lag before some stopping device (eg. magnet) receives said detection signal and acts to stop the movement of the camera lens, the moving speed of a camera lens interlocking with the scanning mechanism becomes slow, if the scanning speed of it is slow and discrepancy of the position of stop of the camera lens lessens. And when the main illuminating light of the subject is a fluorescent lamp or something, the discharge cycle of the fluorescent lamp overlaps the output of a light receiving element. By this, the position of maximum value (or minimum value) is deviated and an error will occur on the originating time of the detection signal. And when a camera lens is stopped by thrusting a click-like member into a saw tooth-like restraining portion, a camera lens is to be set at the same place, even if the click-like member thrusts into any portion of a valley of one saw tooth, but in the one in which the stop position of a camera lens is determined stagewise, if the moving speed of a camera lens is made slow, the error of the originating time of detection signal due to light of a fluorescent lamp as mentioned above, is sufficiently small; there is obtained the effect that the range within a valley of one saw tooth is obtained, and a camera lens can be stopped at an accurate stage, that is, the position of a camera lens determined by a detection signal. Further, since a camera lens is fairly heavy, a great inertia applies to a source of energy, whatever way it may be operated in (such as spring or motor). Therefore, the operation becomes unstable and further when a camera lens is stopped, there arises a problem of photographing blur due to vibrations, etc. However, if the moving speed of a camera lens is made slow, the above defect can be eliminated.
As mentioned above, in automatic focus control, it can be said that the slower the scanning speed of scanning mechanism of a photoelectric distance measuring instrument and the speed of a camera lens interlocking with the working, the higher the accuracy rises, but since a photographer is to release a shutter watching for a good chance of taking photographs, time required for automatic focus control naturally has its own limit. Empirically, that seems to be about 100 ms. In short, an ordinary photographer holds a camera in the photographing state for about 100 ms. after pressing a shutter release button, but he thinks that photographing is over if more time than that has passed. Of course, if more time than this, is needed in focus regulation, a chance of photographing will be lost and the desired photograph can not be taken. For this reason, there arises a limitation that a scanning speed of scanning mechanism of a photoelectric distance measuring instrument cannot be dropped to a given point of below.